Method and a device for pickling of stainless steel

ABSTRACT

PCT No. PCT/SE94/00406 Sec. 371 Date Dec. 25, 1995 Sec. 102(e) Date Dec. 25, 1995 PCT Filed May 4, 1994 PCT Pub. No. WO94/26959 PCT Pub. Date Nov. 24, 1994A method and a device for performing the method of electrolytic pickling of a metal strip. The metal strip continuously passes through an electrolyte bath which has an electrolyte circulating through a closed system. Crevices are formed on both sides of the metal with electrodes which are located above the top crevice and below the bottom crevice. The electrodes are of opposite polarity and are chemically resistant to the electrolyte. An electrical current is passed from one electrode, through one crevice, through the metal, through the other crevice and to the other electrode which results in the electrolytic pickling of the metal.

This application is a 371 continuation of PCT/SE94/00406 filed May 4,1994.

This application is a 371 continuation of PCT/SE94/00406 filed May 4,1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for removal of oxide layers,chromium depleted zones and the like in pickling of a metal, in thefirst place stainless steel, more particularly high alloy stainlesssteel in the form of plates or strips, continuously passing in anelectrolytic bath. The invention also relates to a device for performingsaid method.

2. Decsription of the Prior Art

The development of new high alloy stainless steel, both austenitic andferritic-austenitic steels, requires new or improved pickling methods.The methods used up to now are principally the same used for somedecades for lower alloyed stainless steels.

A known pickling technique is pickling in different mineral acids ormixtures of acids. Further electrolytic pickling in neutral saltsolutions is used, see the Swedish patent 205 105.

Electrolytic pickling in mineral acids or mixtures of acids is used toget a fast pickling in continuous annealing/pickling lines, where theprocess control is related to the strip speed, see report by S Owada etal, A new electrolytic descaling in HNO₃ --HCl acid for development offunctional stainless steels; in Proc. International Conference onStainless Steels, 1991, Chiba, ISIJ, p 937. Electrolytic pickling withalternating current in mineral acids or mixtures of acids is also knownaccording to the Swedish patent 132 298.

For high alloy stainless steel the mentioned methods have problems bothin obtaining a clean surface without any annealing oxide and in removingthe chromium depleted zone, about 2-20μm deep, below the annealingoxide. For the new high alloy stainless steels and their demandingapplications it is of outmost importance that the surface has theproperties of the alloy, that the composition is perfect in the surface,thus that the chromium depleted zone has been removed. The lowerchromium content in the surface zone means a considerable deteriorationof the surface properties, e. g. the pitting resistance, compared to theproperties of the bulk material below the surface. The critical pittingtemperature in potentiodynamic test in 1M NaCl is for several high alloysteels over 90° C. in the bulk material, but in presence of a chromiumdepleted zone in the surface the critical pitting temperature can beonly 70° C. in the surface. It is known that if pitting once has startedin the deteriorated surface zone, the pitting attacks will continue downto material with the right composition. Grinding has been tested toremove the chromium depleted zone, but causes microcrevices in thesurface and impurities from the grinding belt and thus deterioratedcorrosion properties in the new surface.

The industry has even been forced to accept a certain chromium depletionin the surface of high alloy steels due to the pickling problems, seereport by J F Grubb, in Proc. International Conf. Stainless Steels,1991, Chiba, ISIJ, p 944.

The industry has also tried to and still tries to solve the processtechnical problems by combining several different pickling methods in aproduction line, e. g. electrolytic pickling in neutral salt solution,followed by mixed acid pickling. Further, mechanical stages as shotblasting, brushing and possibly grinding are often included.

For mixed acid pickling, in which a high speed (e. g. 30 m/min) is usedin continuous processes for passing through the furnace, long picklingbaths, high acid concentrations and high temperature are required tomanage to achieve an acceptable pickling effect. This means a greatstrain on handling and environment. Big volumes of acids, HF and HNO₃,and big volumes of air with reaction gases, nitrogen oxides , must behandled in refining and retardation stages. For the high alloy stainlesssteels a final pickling according to this method cannot manage thepreviously mentioned problem with chromium depletion in the surface.

Electrolytic pickling in neutral salt solutions gives an improvedenvironmental technique, but the process is only used to break up oxidelayers. Final pickling must be performed as mixed acid pickling, wherethe effect of the process is limited according to the paragraph above. Ametallurgical drawback for high alloy stainless steel is also thatpitting can occur in the electrolytic pickling stage. In application ofthe electrolytic process, the material is the centre conductor and thematerial passes a series of electrode pairs comprising in turnanode/anode , cathode/cathode, anode/anode etc. Thus, the electrodepairs have mutually the same polarity and voltage and they are placed atboth sides of the strip travelling through the bath.

For electrolytic pickling in acid it is known that in a laboratoryscale, where the voltage between the electrolyte and a steel specimen iscontrolled by a reference electrode, the pickling process can becontrolled to selective pickling of the oxide layer and the chromiumdepleted zone , respectively. This method cannot be used in industrialscale for a continuous pickling process for strip with a heterogeneouschemical potential, because material with oxide would be fed into thepickling bath simultaneously with a completely pickled material beingfed out of the bath.

Electrolytical pickling by alternating current and mineral acids ormixed acids as electrolyte is a known old technique described in theSwedish patent 132 298 among others. In examples a method used forstatic pieces to be pickled, e. g. plates hanging in the acid, isdescribed. One of the plates can be one of the electrodes, whichsubsequently will also be pickled. It is also mentioned in thedescription to use several plates in the same electrolyte, alternatinglyconnected to the power source. Further, there are examples, where thecentre conductor principle with liquid contact between electrode andplate is used. However, they recommend that the strip should not be usedas a centre conductor and liquid contact, instead the product to bepickled (the strip) should be connected as an electrode. There is notany special description of how to do this connection.

For a person skilled in the art it would be natural to connect the powersupply via sliding contacts, metal rolls or the like. The mentionedpatent says that the electrode material is preferably stainlessmaterial. Then a problem is that stainless electrodes, which arepreferred in said patent, are consumed in the same proportion and speedas the pickled product (the strip) and a high consumption of electrodeswill give problems in continuous processes. Furthermore acid is consumedfor the pickling of electrodes. Further a non-negligible potentialdecrease will be obtained between stainless electrodes and theelectrolyte, which gives problems with increased temperature of theelectrolyte, contrary to what is stated in the mentioned patent.

A known technique for continuous passage of strip horizontally throughelectrolytic baths is to use open baths, where the strip is pressedbelow the electrolyte surface by guiding rolls, which must be isolatedby rubber, plastic or the like. The open baths involve environmentalproblems. As the strips can be more or less unflat, both longitudinallyand widthwise, and their surfaces can have certain defects andirregularities, the rolls are exposed to both chemical attack andmechanical wear and all this requires exchanges of the rolls andsubsequently production stops.

Another known technique is to feed the strip into an opening in the wallof the bath and tighten from the inside with couples of opposed steelrolls, dressed with rubber or plastic etc., at the bath wall. As aconsequence, the rolls , which must have a great diameter to level anyknobs and dents in the strip, are closely pressed against the stripsurface to tighten any leak of strong process solution through the wallopening or in the crevice between the rolls and the strip. Further, theroll coating often is exposed to solutions of high temperature, whichcauses a faster break down of the coating. Thus, the wear of the coatingof the rolls can be big, and the exchanging of the rolls causes longprocess stops and breaks of production flows.

In common, the prior art bath constructions have tightening means, beingfully or partly immersed in the process solution and having the functionto guide the position of the strip. In summary, the greatest problemswith the known technique for passage of a strip in a pickling bath are

immense wear of the tightening means (the roll coating) bothmechanically, due to the surface condition of the strip and the knobsand dents in the strip surface, and chemically, due to strong processsolutions, e. g. strong acids, at high temperatures

drift stop for exchange of rolls.

The object of the invention is to provide a total solution of theprocess technical problems in pickling of stainless steel strips,particularly of high alloy stainless steel, which pass continuously inan electrolytic bath, and to attain both a clean pickled surface and thecorrect surface composition and still meet both productional andenvironmental demands as to prevent any leak of strong process solutionsand reaction products. The invention is a solution of these problems.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a device andmethod thereof for electrolytic pickling of a metal. The device includesan electrolyte bath having an electrochemical cell and an electrolytecirculating through a closed system. The electrochemical cell has twocell halves and two electrodes, each cell half containing one of theelectrodes. The electrodes have opposite polarities and are chemicallyresistant to the electrolyte. Viewing the device from the bottom on up,there is located one of the electrodes, a crevice, the metal, anothercrevice and the other electrode. Preferably, means for tightening themetal as it passes through the electrochemical cell are also provided.The electrolytic pickling is accomplished by passing an electricalcurrent from one electrode, through the bordering crevice, through themetal, through the other crevice and to the other electrode.

The invention can be used in a separate pickling line for strips havinguncoiler/recoiler or for plates fed into the pickling device via aroller table. The invention can also be a part of a continuousrolling/annealing/pickling line, alternatively an annealing/picklingline.

As several stages in a complete pickling line are well-known , e. g.rinsing and drying, the electrochemical cell and the electrolyte flow,only, are shown in the following description of embodiments. A number ofcells according to the invention should be placed in a series in a stripline to manage pickling at a speed equal to that of the other processstages. The size of the cells can also be varied. In a pickling lineconsisting of more cells, fully individual parameters (electrolytes,voltage, current density, direct current or alternating current) canalso be used in different cells according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

One embodiment of the device according to the invention is shownschematicly in the drawings, where FIG. 1 shows a section, in thefeeding direction of the strip, through a cell for electrolyticpickling. FIG. 2 shows a section of the tightening means and FIGS. 3Aand 3B show two sections of the cell perpendicularly to the feedingdirection of the strip and here it is apparent how the electrolytecirculates in the cell.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following components are shown in FIG. 1, an electrochemical cellconsisting of two cell halves 2,3, made of chemically resistantmaterial, above and below a strip 1. The cell halves contain two platesof graphite electrodes 4,5 and tightening means 6-9, which tighten theinlets and outlets of the strip in the cell. The electrolyte is suckedinto the cell via transverse inlet channels 11,12 and is sucked furtherthrough a thin crevice 15 above the strip and a thin crevice 16 belowthe strip and leaves the cell via transverse outlet channels 13,14.Screws 17, 18 keep the graphite electrodes in place and connect themelectrically to a not shown alternating current power supply via cable19 to one pole and via cable 20 to the other pole. Outside the cellthere are guidance rolls 21-24 to keep the stretched strip 1 positionedbetween the cell halves 2,3. It should be noted that the figure showsjust one screw and cable per graphite electrode, but to transfer highcurrents, a great number of screws/cables is required.

FIG. 2 shows a section of a couple of tightening means 6,7, preferablymade as strips, at the inlet of the strip in the cell. There arecorresponding tightening means 8,9 at the outlet of the strip (see FIG.1). The tightening strips are made of strandblown rubber with a straightprofile in the centre and one edge 35,36 reinforced against wear causedby the metal strip. The other edge 33,34 has a round profile with a holein the centre to fit into a track 37,38, in the cell half 2,3 forholding of the list, resp. Springs 31,32 are made as straight, densespiral springs and by their assemblage the tightening strips are alwayspressed against the strip 1. By this shape of the tightening means 6-9,it has surprisingly turned out that not even strips with bad flatness byknobs, dents and surface defects are able to open the tightening meansto an extent giving any problems with leak of acid. The wear in points35,36 and correspondingly at the outlet, has also turned out to besmall, in spite of the passage of several kilometers of strip per hour.At their outlet through the cell wall, the tightening strips pass endpieces, which are not shown in the figures. The tightening strips can beexchanged, during running of the pickling line, by pulling fresh stripinto tracks 37,38, from a supply roll, not drawn, beside the cell, bymeans of the old worn strip being pulled out of the cell and being cutoff. The number of springs per tightening strip may be 100 per meter,and it has turned out that the springs, by their assemblage, are notcausing any problems when exchanging the tightening strip.

FIG. 3 shows the flow of liquid through the cell. FIG. 3A shows thelevel tank 25 with electrolyte and a coarse connecting tube 26 connectedto the lower cell half 3. Via the inlet channels 11, 12, resp., theelectrolyte passes into the crevices 15,16 between the graphiteelectrodes 4,5 and the strip 1. FIG. 3B shows how the electrolyte flowsout of the cell via outlet channel 13 and 14 and then the electrolytefalls freely in a coarse tube 27 connected to a centrifugal pump P andfurther back to the level tank 25. The electrolyte can also fall freelydown into a big supply 29 below the cells and can then be pumped to thelevel tank 25 through connection tube 28 via pump P1. To preventoverflow there is an overflow drain tube 30 mounted in the level tankfor return flow to the supply tank. A fan outlet 10 is connected to astrong fan giving a big negative pressure in the cell, and thereby sucksthe electrolyte in and makes the electrolyte level in the cell higherthan the level in the level tank 25, and removes all formed gases.Tightenings 40, 41 at the cell edges parallel to the feeding directionof the strip are schematicly drawn and are shaped as bellows. Thisallows a variation of the electrode distance in the cell.

The electrolytic pickling according to the invention is initiated byfeeding the strip into the cell via guidance rolls 21, 22 , see FIG. 1,further between the cell halves 2,3, which can be separatedautomatically , so a big crevice is obtained when feeding in a newstrip, and further out between the guidance rolls 23,24. The cell halvesare brought together and the pump P (alternatively P1) is started andthereafter fans are started for evacuation of the cell via the fanoutlet 10. The electrolyte now begins to circulate through the cell,when via the connection tube 26 it is sucked into the cell up to a drawnequilibrium level in the outlet channel 13, and then it falls down intothe tube 27 and is pumped back to the level tank 25. Then thealternating current to the graphite electrodes is switched on andelectrolytical pickling of the two surfaces of the strip starts. Thestrip is then fed through the cell continuously. Gas bubbles and sludge,formed at the pickling, are driven away from the surfaces of theelectrodes and the strip by the heavy electrolyte flow and can beseparated out in filters or the like. The electrolyte flow chills alsoand removes reaction heat from the process.

If strips narrower than the electrodes are to be pickled, currentshielding plates can be disposed at the edges of the strip to preventthe current to pass directly between the electrodes which else wouldgive effect losses.

The principle of the electrolytic pickling in acid with alternatingcurrent according to the invention is that the alternating current goesfrom the graphite electrode to the strip via the upper electrolyte andpasses perpendicularly straight through the strip in its thicknessdirection and further via the lower electrolyte to the opposite graphiteelectrode. The two electrolytes are separated from each other by thestrip and if necessary by isolating plates.

By the invention it has surprisingly turned out that an increasedpickling effect in pickling of strips is obtained using electrolyticpickling in mineral acids or mixtures of them with alternating currentor polarity reversed direct current. A surface without chromiumdepletion is obtained, if the principle of passing an alternatingcurrent or polarity reversed direct current straight through the stripin its thickness direction is used, and , instead of what is said in theSwedish patent 132 298, the combination of graphite electrodes andliquid contact is used. The graphite electrodes in combination withalternating current have also the advantage that acid will not beconsumed for pickling of the electrode which would be the case usingelectrodes of stainless steel. According to the invention it has alsosurprisingly turned out that the wear of the graphite electrodes incombination with alternating current is very low, quite contrary to whatis said in EP-A1-137 369, where it is also obvious from the wiringdiagram that the alternating current is not to be passed perpendicularlythrough the strip, but along the strip to auxiliary help electrodes.

It is known by U.S. Pat. No. 4,276,133 and EP-A1-209 168 that wire canbe pickled electrolyticly and continuously in acids at a high currentdensity (200 A/dm²) in partly closed systems. The method according tothese documents is that the current is not passing in the thicknessdirection, but passes from anode to wire, follows the wire a certaindistance and then leaves the wire to go to the cathode. However, forcontinuously travelling strips, several of the parameters (materialarea, total current, acid leak at in- and outlets of the cell,non-flatness etc) will be at least 100 times bigger than for wire, andas a consequence such a wire pickling technique cannot, in practice, betransferred to a corresponding technique for strip pickling.

Abstract of JP-A-60-135 600 shows a construction with direct current,where the current is led in the thickness direction of the strip, andthe strip is pickled alternating on its two surfaces between electrodepairs, where the pairs must be separated from each other in the feedingdirection of the strip to prevent the current to pass directly in thebath between, in the feeding direction of the strip, adjacentelectrodes. This causes problems with unnecessarily long total length ofpickling lines. Further such a construction is not applicable to mineralacid, which has about 5 times higher conductivity than salt solutions,and then a still bigger separation between different electrode pairs inthe feeding direction of the strip would be required. The document doesnot say how to obtain a high current density, from a process technicalpoint of view as in the present invention, in pickling of stainlesssteel in mineral acid.

Further, surprisingly, the device according to the invention gives asolution of the problem with acid leak at in- and outlets of thecontinuously travelling strip, which can be 2 m wide and furthermore canhave more or less dents and knobs.

It is particularly surprising as U.S. Pat. No. 4,276,133 shows that theynot even for wire have thought of a sufficient tightening at continuouspassage through the cell wall, but use overflow protection, which isreasonable for the relatively small overflow volumes in pickling ofwire. For pickling of strip this principle is not reasonable due to thebigger overflow volumes.

The construction with graphite electrodes in cell halves means also thatthe active volume of acid is considerably smaller than in conventionalpickling in mixed acid. A system for transport of acid in narrowcrevices in pickling of mild steel strips is described in EP 0 276 384.However, the system is only intended for chemical pickling of mild steelin acid.

It should be observed that the pickling effect (the volume of materialremoved by pickling) is proportional to the current density (A/dm²). Theinvention allows high currents to pass through the strip, in spite ofthe fact that graphite with a conductivity about 350 times less thanthat of copper, must be chosen due to the acid environment and corrosionconsiderations. The short way of the current through the crevices withelectrolyte and the supply of current to the graphite electrodes frommany points through the thickness direction, give a low voltage decreaseand thus just small effect losses are obtained. An industrial picklingline for neutral pickling is often supplied by 20 V and then a currentof 20 000 A will pass the strip, while according to the invention, only8 V will give a current of 50 000 A. In both the cases the effect willbe 400 kW, but 2,5 times higher pickling effect is obtained by theinvention.

The technical effect is also shown by the following examples and bythese and the previous description the effect of the invention can besummarized:

As a whole, the invention can be considered as a challenge of thenatural laws, where it has surprisingly turned out that it is possibleto considerably increase the pickling effect by combining a fastcirculating flow, produced by evacuation, of electrolyte in anelectrolytic cell and supply of current in the thickness direction ofthe strip, and that, at the same time, it is possible to overcome theenvironmental and safety problems related to continuous passage of verybig lengths of stainless steel strips in in- and outlets to strong acidsat a high temperature. A further environmental effect is that theinvention, by the use of sulphuric acid, completely eliminates theproblem with nitrogen oxides from use of nitric acid and the problemwith handling of the fluoric acid.

EXAMPLE 1

For a high alloy stainless steel with 20% Cr, 18% Ni, 6% Mo and 0,2% Nstrips were produced with a thickness of 0,8 mm. After annealing thestrip was pickled electrolyticly in a neutral salt solution of Na₂ SO₄ ,whereafter the strip passed a brushing stage , where rests of oxide wereremoved. Final pickling was performed in mixed acid (5% HF/20% HNO₃)Samples from the strip were examined in an electron probe microanalyser(EPMA) and the chromium content in the surface was determined with thisinstrument. The surface structure had relatively smooth scratches by thebrushing and between the scratches there were pickled areas where thegrain structure was clearly apparent. The chromium content in thescratches was 19,88%, whereas it was only 16,58% in the pickled areas,thus locally, the surface was strongly chromium depleted. According tothe invention a test plate of the strip material was pickled for 55 s at200 A/dm² and 8 V in 30% H₂ SO₄. Surface analysis with electronmicroprobe analyser was performed and the surface content was 19,9%,thus no remaining chromium depletion. The structure was now uniformwithout any overpickling.

The pitting properties of the surface was tested in 1M NaCl, accordingto ASTM G61, with the so called Avesta cell. The chromium depletedspecimen had a relatively low CPT (Critical Pitting Temperature) of 70°C., whereas the specimen pickled according to the invention had CPT 92°C. Ground bulk material, in both the cases, had CPT 92° C.

EXAMPLE 2

A conventional pickling line for 1,6 m wide stainless steel strips and astrip speed 10 m/min contained a neolyte pickling unit and 3 acid bathswith HF+HNO₃, each 20 m long, and a shot blasting machine. The totallength of the full pickling line was 90 m. To double the production thestrip speed would be increased to 20 m/min and a calculus was performedfor a new pickling line based upon test results from full scale tests ofa pickling line according to the invention. With only 20 cells accordingto the invention and ditto roll pairs between the cells, a completepickling line could be calculated to have a length of only 30 m (about1/3 of that of the previous line, but with the double capacity), andwhere the environmentally dangerous mixture of 5%HF/20%HNO₃ could bereplaced by 30% H₂ SO₄ . The cost of the investment was calculated to behalf of that of the conventional, previous technique and permittance forthe production could be obtained from the environmental authority inspite of the production being doubled.

EXAMPLE 3

Full scale tests with a cell according to the invention was performedwith the following parameters and were compared with conventionalpickling.

    ______________________________________                                                 The invention    Conv.tech                                           ______________________________________                                        Material   AISI 304 AISI 3   254SMO 254SMO                                    Width, mm  1500     1200     900    900                                       Thickness, mm                                                                             3        2        1      1                                        Pickling time,sec                                                                        15       20       60     120                                       Voltage, V  8        8        8     --                                        Current, kA                                                                              30       24       18     --                                        Effect, kW 240      192      144    --                                        Acid       Sulphuric acid     HF + HNO.sub.3                                  Acid conc. %                                                                             25       30       35     5% + 20%                                  Temp., °C.                                                                        60       60       60      50                                       Acid flow, l/min                                                                         800      700      600    --                                        Oxide rests,                                                                             No       No       No     No                                        visible                                                                       Corrosion test,                                                                          --       --       .sup.  .sup.    70° C.                    CPT                                                                           ______________________________________                                    

After passage of 50 km continuous strip through a test cell according tothe invention with 30% sulphuric acid, 60° C., the wear of thetightening strips was measured and was 0,1 mm in the points 35, 36 ,which gives a running time of about a month and the exchange of thetightening strips is performed in some minutes without having to stopthe pickling process. In conventional technique the process must bestopped and emptying of baths and exchange of rolls take several hours.

The test cell according to the invention had no leak of electrolyte. Noscratches were formed in the strip surface by the tightening strips.

EXAMPLE 4

Process technically and from safety point of view, the cells accordingto the invention have been tested how fast the cell can be emptied andopened if strips with mechanical defects, welded joints etc must pass ina continuous annealing/pickling line. There are two cases, demands forpickling with limited strip speed and totally interrupted pickling,resp. It has then turned out that the system easily allows an increaseof the electrode distance because the tightening strips 6-9 and thebellows 40,41 allow bigger distance between the cell halves 2,3. Totalemptying of the acid in the cell has surprisingly turned out to beperformed in less than a second, in spite of the acid flow being up to1000 l/min while pickling is going on. The evacuation via outlet 10 isswitched off and the electrolyte streams down into channel 27 or back tothe level tank 25 and the process is stopped immediately.

What is claimed is:
 1. A method of electrolytically pickling of a metalin the form of plates or strips which comprises the steps of:a)providing an electrolytic bath containing an electrochemical cell and aelectrolyte circulating in a closed system, said electrochemical cellcomprising two cell halves containing an electrode in each; b) forming acrevice between the metal and each electrode; c) passing saidelectrolyte at an elevated speed through each of said crevices; and d)passing a controlled electric current through one of said electrodes,passing current through said electrolyte, through said metal in thethickness direction of said metal and through the electrolyte to theother electrode.
 2. The method according to claim 1, wherein said metalcomprises stainless steel.
 3. The method according to claim 2, whereinsaid stainless steel comprises high alloy stainless steel in the form ofa strip.
 4. The method according to claim 1, wherein said electricalcurrent comprises a current selected from the group consisting ofalternating current and direct current.
 5. The method according to claim1, wherein said electrical current has a current density of at leastabout 150 A/dm².
 6. The method according to claim 1, wherein saidelectrical current has a voltage of about up to 8 V.
 7. The methodaccording to claim 1, wherein said electrolyte bath comprises a mineralacid.
 8. The method of claim 7 wherein said mineral acid is selectedfrom the group consisting of HF, HNO₃ and H₂ SO₄.
 9. The methodaccording to claim 1, wherein said electrolyte bath comprises sulfuricacid.
 10. The method according to claim 9, wherein said sulfuric acidhas a concentration of about 10 to 40 percent in volume.
 11. The methodaccording to claim 1, further comprising means for tightening said metalas it passes through said electrochemical cell.
 12. A device forelectrolytic pickling of a metal strip, said pickling devicecomprising:an electrolytic bath, said electrolyte bath containing anelectrochemical cell and an electrolyte circulating through a closedsystem; means for feeding said metal through said bath; saidelectrochemical cell consisting of two cell halves and two electrodes,each cell half having one of said electrodes; said first electrode beingsituated so as to be above said strip and forming a first crevicetherebetween; said second electrode being situated so as to be belowsaid strip and forming a second crevice; each electrode having oppositepolarities and being chemically resistant to said electrolyte; wherebywhen said strip is fed in between said electrodes and an electricalcurrent is passed from said first electrode, through said first crevice,through said metal, through said second crevice and to said secondelectrode there is a resulting electrolytic pickling of said metal. 13.A pickling device according to claim 12 further comprising means fortightening said metal as it passes through said electrochemical cell.14. A pickling device according to claim 13, wherein said tighteningmeans is spring loaded.
 15. A pickling device according to claim 13,further comprising bellows to separate said cell halves during saidelectrolytic pickling process.
 16. A pickling device according to claim13, wherein said tightening means comprises at least four tighteningstrips; two of said tightening strips being situated above and belowsaid metal at one end of said pickling device and said other twotightening strips being situated above and below said metal at the otherend of said pickling device.